Contact lenses are commonly placed into two categories, soft lenses and hard lenses. Soft lenses are rubbery and pliable, and they are fitted so as to closely match the shape of the cornea. Because of their softness they are more comfortable for the wearer than are hard lenses and because of their tight fit they have to be oxygen permeable in an amount sufficient to meet the needs of the cornea if worn for extended periods.
Hard lenses on the other hand have to possess excellent dimensional stability and are fitted so as to sit on the cornea only with the center of the lens, leaving enough space between their edges and the eye to allow good tear liquid exchange and thereby oxygen supply to the cornea. Although hard lenses are less comfortable to wear, they can be fabricated with greater precision and are preferred to correct, for instance, astigmatism. For both types of lenses excellent wettability by the tear liquid is important. This is especially true for soft lenses; soft lenses which cannot freely rotate on the eye and which are not easily wettable can become stuck to the cornea--which also is hydrophobic--and difficult to remove, sometimes only with injury to the eye, due to the suction-cup effect. For this reason commercially successful soft lenses are primarily of the hydrogel type, most commonly made from poly(2-hydroxyethyl methacrylate, HEMA), from HEMA copolymers, or from vinylpyrrolidone copolymers. These contact lenses generally contain from 38-75% water and thus water content is primarily responsible for their softness, their oxygen permeability (DK) of about 8 to about 30 barrers and their good physiological response.
On the negative side, the high water content makes a rather weak polymer, and oxygen permeability cannot therefore be further increased by raising water content or cutting thinner lenses. Oxygen permeability of 20-50 barrers for hard lenses and up to 100 barrers for soft lenses are desirable and required for extended wear contact lenses. In addition, hydrogel contact lenses have to be regularly sterilized.
These drawbacks could be overcome if silicone rubber could be used as contact lens material; it is strong, comfortably soft and polysiloxane is the polymer with one of the highest known oxygen permeability. Unfortunately polysiloxane is also very hydrophobic and imparts this hydrophobicity to polymers which contain a substantial amount of it. This hydrophobic character also leads to excessive deposition of protein and/or lipids on a contact-lens surface.
A number of methods have been used to make polymethylmethacrylate (PMMA) and polysiloxane based contact lenses more wettable. U.S. Pat. No. 4,131,696 describes the treatment of PMMA contact lenses with aqueous, colloidal suspensions of inorganic hydroxides. Plasma-treatment in a glow discharge chamber using water, hydrazine, ammonia, formamide, ethylene diamine and similar compounds is used to hydrophilize PMMA and silicone rubber and is described in U.S. Pat. No. 3,952,178. A similar process using gases such as oxygen, nitrogen, NO.sub.2, N.sub.2 O, N.sub.2 O.sub.3, NO, SO.sub.2 and CO is described in FR No. 2,483,310. U.S. Pat. No. 4,139,660 describes the treatment of hydrophobic surfaces with halides of elements from group IV of the Periodic Table, followed by an aqueous rinse. U.S. Pat. No. 4,055,378 describes a treatment by activated oxygen ions of an SiO.sub.2 -filled silicone lens, by which the SiO.sub.2 filler becomes exposed to the surface and/or organic silicone groups are oxidized to SiO.sub.2 to thereby render the surface more wettable. U.S. Pat. No. 4,062,627 and U.S. Pat. No. 4,099,859 describe free-radical initiated grafting reactions of N-vinylpyrrolidone or hydroxyalkyl(meth)acrylates on the surfaces of silicone rubber contact lenses.
Every one of these methods suffers from one or several drawbacks. For example, plasma treatments and oxidizing treatments of PMMA and silicone surfaces produce only very thin modified surface regions and therefore these treatments do not provide sufficiently permanent wettability. If the effect wears off, it can only be restored by a new and expensive repetition of the original treatment. Grafting methods involve the use of a swelling monomer on a precision-cut contact lens, which can easily lead lo permanent distortions of its optical properties. In addition, by the very nature of free-radical initiated grafting reactions quality control is very difficult and the whole process is cumbersome. The grafting polymerization must be conducted at temperature levels and periods of time under which the vinyl monomer is capable of diffusing into and swelling previously grafted polymer chains, thereby producing irregular regions of proliferating grafting by a positive feedback effect. In other words, after the very first grafting to the hydrophilic silicone surface has been accomplished the new surface is much more absorptive to the monomer. The monomer is thermodynamically compelled to swell the grafted hydrophilic polymer and further irradiation will produce additional grafting in regions already reached in the polymer. Thus, the process is one of positive feedback and can lead to gross differences in amount of grafted hydrophilic polymer per unit area to silicone substrate. Upon subsequent immersion in water the more grafted regions swell to the greater degree, thereby potentially distorting the interface and producing undesirable optical dispersion.
It has now unexpectedly been discovered that if glycidyl groups containing contact lenses are treated under mild aqueous conditions with a variety of primary or secondary amines, thiols, thiourea, sulfite or thiosulfate, a permanently wettable surface is obtained without impairing the clarity or the optical quality of the lens. It is a further advantage of the process of this invention that it is exceedingly simple and very easy to control and, should the effect ever wear off, very safely and easily repeated by the wearer of the lens at home. It is a further objective of this invention to use the described surface-treatment of glycidyl groups containing contact lenses during the routine contact lens cleaning procedures carried out by the contact lens wearer. We speculate that the high quality and distortion-free nature of the final contact lens is a result of the single-step nature of the reaction. Step-reactions, of which polycondensation is a prime example, are much easier to control than are chain-reactions, where a very rapid growth reaction generally leads unpredictably to very high molecular weights. Single-step reactions also assure excellent compatibility of the treated surface with the underlying bulk polymer whereas the high molecular weight of free-radical grafted chains leads to phase separation and thereby optical haziness.
Glycidyl acrylate and glycidyl methacrylate as components of silicone-free hydrogel contact lenses have been specifically described in U.S. Pat. Nos. 3,787,380, 3,758,448, 3,772,235, 3,947,401; in several other patents it is disclosed as a possible component of the monomer mixture. In all instances the purpose of using this monomer is to influence water uptake and crosslinking reactions. U.S. Pat. Nos. 4,182,725, 4,182,723 and 4,169,119 describe silicone rubber lenses made wettable by inclusion of small amounts of glycidyl(meth)acrylate polymers, without additional surface treatment. Although an increase in wettability has been shown in these patents, this increase is insufficient to fulfill the requirements of a silicone-based soft contact lens having a surface of sufficient wettability to provide for adequate patient comfort.
In Canadian Pat. No. 1,099,546 glycidyl methacrylate is used as part of a hydrogel-contact lens formulation to introduce rigidity.
U.S. Pat. No. 4,338,419, in relevant part, discloses the complete removal of residual uncrosslinked oxirane units present as a minor impurity in a crosslinked hydrophilic hydrogel material formed of a polymerized comonomer mixture initially containing not more than 5% oxirane monomer as a crosslinking agent, and exemplifying less than 1% by weight thereof in the initial monomer mixture, by reaction with a thiol. No increase in the hydrophilicity of the surface of the lens materials is reported.
It is an object of the present invention, in clear contradistinction to the prior art, to provide normally hydrophobic oxirane containing contact lenses with a substantially increased hydrophilic surface, i.e. to increase the wettability thereof, by incorporating a hydrophilic inducing amount of a water soluble organic amine, alcohol, thiol, urea or thiourea, or a sulfite, bisulfite or thiosulfate - oxirane reaction product on the surfaces of such lens.
It is a further object of the present invention to provide a method to correcting vision defects of a refractive nature by placing in the eye of a patient in need of the same, a contact lens made of such material containing the aforementioned surface modification.
These and other objects of the instant invention are more fully set forth in the following detailed description.